New Astronomy at the New Year (GW170104)

January 4 holds a special place in the hearts of scientists — it is Isaac Newton’s birthday (*). Newton stood at the crossroads that led to modern science, and astronomy in particular. He was the first person to build a workable reflecting telescope, a design that now bears his name and for the past 4 centuries has been the dominant type of telescope used by amateurs and professionals alike. Newtonian telescopes have revealed much about the Cosmos to our wondering minds. Newton was also responsible for the first formulation of a physical law that describes the working of gravity, called the Universal Law of Gravitation. Today we use the Universal Law to launch satellites, send astronauts into orbit, convert the force of your feet on the bathroom scale into your “weight“, and a thousand other applications. There is much to celebrate every January 4.

But on January 4, 2017 the Cosmos celebrated with us, singing in the faint whispers of gravity itself. On January 4, the signal of two black holes catastrophically merging to form a new bigger black hole washed quietly across the shores of Earth, carried on undulating vibrations of space and time. You were very likely unaware of this cosmic event — it happened at 4:11:58.6 am in Chicago. It was a Wednesday morning, and I imagine most people were blissfully asleep. But two of the grandest pieces of experimental apparatus ever built by humans were paying attention – the twin LIGO detectors in the United States. For only the third time in history, a gravitational wave signal from the deep Cosmos was detected here on Earth.

The signal was the signature of two black holes (a “black hole binary,” in the lingo of the astrophysicists) merging to form a new, bigger black hole. The black holes, by definition, emit no light themselves. However, astronomers know that black holes can often be surrounded by swaths of interstellar gas. The intense gravity and motion of the black holes can stir the gas into a violent froth that can emit light. At the time of the event, the LIGO team sent out alerts to astronomers around the world, who turned their telescopes skyward looking for a tell-tale signature of light bursting from the energized gas. Our best estimate of the location of the event was canvased by 30 groups, in many different kinds of light ranging from radio waves, to optical light, to gamma rays. No tell-tale emissions of light were seen. The only way we were aware of this event is from the LIGO detectors themselves.

An artist’s impression of two black holes insprialling, near merger. [Image by Aurore Simonnet, SSU E/PO]

The Gravitational Wave Signal. We call the event GW170104, named for the date it was detected. The signal from the black holes registered first in the LIGO detector outside Hanford, Washington, and 3 milliseconds later registered at the LIGO detector outside Livingston, Louisiana. All told, it only lasted about 0.3 seconds. The signal exhibited the characteristic chirp shape expected of compact binaries that spiral together and merge — a long sequences of wave peaks that slowly grow in strength and get closer and closer together as the black holes spiral together.

Comparison of the chirp waveforms from the first 3 detected gravitational wave events. LVT151012 was a very quiet event that was not strong enough for LIGO scientists to be confident it was a pair of black holes. [Image: LIGO Collaboration]

During the early inspiral phase of GW170104, where the black holes are independent and distinct, the heavier black hole of the pair was 31 times the mass of the Sun, and the smaller black hole was 19 times the mass of the Sun. Ultimately, they reached a minimum stable distance (in astrophysics lingo: the “innermost stable circular orbit“) and plunged together to form a new bigger black hole. When that plunge happened, the gravitational wave signal peaked in strength, and then rang down and faded to nothing as the black hole pulled itself into the stable shape of single, isolated black hole. For GW170104, this final black hole was 49 times the mass of the Sun.

All of this happened 3 billion lightyears away, twice as far as the most distant LIGO detection to date. Perhaps these numbers impress you (they should) — they tell the story of events that happened billions of years ago and in a place in the Cosmos that neither you, nor I, nor our descendants will ever visit. We add them today to a very short list of astronomical knowledge: the Gravitational Wave Event Catalogue, the complete list of gravitational wave signals ever detected by human beings. There are only three.

The current Gravitational Wave Catalogue, of all known events [click to make larger].

Take a close look at the list. There are interesting similarities and interesting differences between the three events. They are all black hole binaries. They are all at least a billion light years away from Earth. Some of the black holes are heavier than 20 times the mass of the Sun, and some are lighter than 20 times the mass of the Sun. Astronomers use those comparisons to understand what the Universe does to make black holes and how often.

This is the most important thing about GW170104 — it is a small but significant expansion to this very new, and currently, very limited body of knowledge we have about the Cosmos. These three events are completely changing the way we think about black holes in the Cosmos, forcing us to rethink long held prejudices we have about their masses and origins. We shouldn’t feel bad about that — evolving our knowledge is the purpose of science. LIGO is helping us do exactly what we wanted it to do:it is helping us learn.

What do we know?There are many things we are trying to learn from the meager data contained in these three signals. The new signal from GW170104 in particular has tantalizing evidence for the spin of the black holes, and some neat assessments of how close these astrophysical black holes are to what is predicted by general relativity. But I think the most important thing about the event from the perspective of astronomy is this: the black holes are, once again, heavy. GW170104 is the second most massive stellar mass binary black hole ever observed (GW150904was the heaviest).

The masses of known black holes. The purple entries are observed by x-ray telescopes, and represent what we knew about the size of black holes before LIGO started making detections. [Image: LIGO Collaboration]

With the first two events we had one pair of heavy black holes (GW150914), and one pair of lighter black holes (GW151226). There is a great mystery hiding there: where do the heavy black holes come from, and how many are there in the Cosmos? Perhaps they are just a fluke, a random creation of Nature that is possibly unique in the Cosmos. But the detection of GW170104 suggests that this is not the case; we’ve once again detected heavy black holes. The race is on to decide how the Cosmos makes them. The answers to those questions are encoded in the properties of the black holes themselves. How many are there? Are they spinning or not? Are they spinning the same direction as one another? How do their masses compare to one another? GW170104 is another piece of the puzzle, and future detections will help solidify what we know.

How can you help? If you’d like to help the LIGO project out, let me direct your attention to one of our Citizen Science projects: GravitySpy. Your brain is capable of doing remarkable things that are difficult to teach a computer. One of those things is recognizing patterns in images. The LIGO detectors are among the most sensitive scientific instruments ever built; they are making measurements at the limit of our capabilities, and there are all kinds of random signals that show up in one detector or the other — we call them glitches. It is very hard to teach a computer to tell the difference between glitches and interesting astrophysical events, so we have citizens just like you look at glitches and identify them, then we use that information to train the computer. So far citizens like you have helped LIGO classify more than two million glitches, and they put more on the pile every day.

If you’d like to help out too, head over tohttp://gravityspy.org/and try it out; you can do it in your web-browser, or on your phone while you’re sitting on the train to work. We have citizens from kids to retirees helping us out. If gravitational waves aren’t your thing, there are more than 50 other projects in science, arts, history and more at http://zooniverse.org/ you can try out!

A representation of the GW170104 signal, from the scientific paper. These are the kinds of images citizens can classify easily, whereas computers sometimes have trouble. [Image: LIGO Collaboration]

PS: For all of you super-nerds out there, let me point something out if you haven’t already noticed. Suppose you were to parse the name of the signal in the following way: 1701 04. Look familiar? The 4th incarnation of 1701; for the cognoscenti, this event shares the designation of the Enterprise-D. 🙂 Until next time, my friends. Live long, and prosper.

(*) When Newton was born, England had not yet switched to the new Gregorian Calendar, which we use today. They were still using the older Julian Calendar, by which Newton was born on December 25; when converted Newton’s birthday falls on January 4 on the Gregorian Calendar.

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You can read about the previous LIGO detections in my previous posts here:

Many of my colleagues in the LIGO Virgo Collaboration have also written excellent blog posts about the GW170104 event, and the work we do to make gravitational wave astronomy a reality. You should visit their blogs!

Are we finally ready to admit the obvious? By far the best dark matter candidate is decidedly NOT putative ad hoc WIMPs, axions, sterile neutrinos, or anything else in the imaginary particle zoo. Chronic No-Shows.
The best candidate for the dark matter is stellar-mass primordial black holes based on actual observations of 100s of billions of MACHO microlensing objects, the new black hole populations discovered by LIGO, and the astronomical numbers of fast radio bursts and gamma-ray bursts.

It is starting to get a little perplexing that no WIMPs have been detected, and the possibilities are getting squeezed. The MACHO surveys ruled out galactic dark matter being a significant fraction of MACHOs, but were centered on 0.5 solar mass black holes. There was interesting speculation recently that the heavy black holes LIGO is seeing might be good candidates, but I think it needs more consideration and thought. In reality, the “solution” to dark matter may be a mix of all the ideas! We’ll see…

I agree that the dark matter enigma is far from settled, but I would point out a couple of facts.

1. 10-20% of the dark matter is one hell of a lot more than 0 particles.
2. MRS Hawkins has published a paper (MNRAS) showing that, with differing assumptions about the MW Galaxy model and the spatial/velocity distributions of the MACHOs, they could make up to 100% of the galactic DM.
3. Previous estimates of MACHO limits were mostly based on unrealistic delta functions for their mass distribution.
4. I predicted (Astrophysical Journal 322, 34-36) that the DM was in the form of black holes with masses ranging from 0.2 to 35 solar masses.

What we need are minds that are fully open to new and different ideas. It is time for the obsession with particle DM to be dialed back.

It’s a fair question to ponder, ie; do black holes rotate (spin)?
Spinning suggests a definition of clockwise or anti-clockwise as we know it. Maybe they spin forward and back or perhaps just suck as a vacuum would.
It is interesting to ponder why a black hole moves at all. Do they roam around this universe looking for something to enlarge their own selves? How would two black holes ever get together in the first place? If it is possible for two black holes to meet, then it may be possible for a parent of all black holes to be lurking out there somewhere. Progressively sucking in all of the others.
This happened 3 billion lightyears away and that’s a very, very long way.B

B — We have a well defined way to describe spin direction called “handedness.” Let’s use a globe as a stand-in for a black hole. (1) Spin the globe. (2) Pretend you could hold the globe in the palm of your right hand, and line your fingers up on the equator so they point in the direction the globe is spinning. (3) Your out stretched thumb is the direction we describe the spin as being in [in physics lingo, your thumb points “along the angular momentum”].

As for how black holes find each other, that depends on how they form. If you assume they form from exploding stars, then binary stars should have a possible end state that is 2 black holes. If they form from stars getting close together in dense globular star clusters, they are harder to get together in some sense, but they do sometimes and have highly eccentric orbits. These are all outstanding ideas and questions about these systems that have yet to be resolved.

Terrific article, as always. You are SO so good at explaining scientific topics to lay folk (something this humble science writer greatly appreciates). But the Enterprise-D thing … well that’s the icing on this gravitational wave cake. 🙂